What is a slab on grade foundation? is there anything below grade?

 

What I mean by slab on grade foundation is that there's no basement or crawlspace. The slab rests atop crushed gravel which sits on the earth below. Typically the edges are thicker and include an integral footing...a monolithic slab foundation.

 

Does the foundation go down far enough to use it as wall against the new slab? How are you dealing with the grades outside the addition?

 

The foundation walls go down 18" if I remember correctly. I know they're no less than that because the frost line in this area is about 6", and I know the footing has to extend a minimum of 12" below the frost line. It's no deeper than 24". The property slopes considerably down towards where the addition will be, so when the addition is completed, the grade will be sloped as well. I can attach two photos to show how the property looks now, where the addition will go, and a 3D rendering of the addition as it sits on the property. I don't know if there is enough existing foundation wall to use as an attachment point...at least not if the delta elevation bxn top of slabs is to be 24".

In the photo's you can see where the addition will occur and the natural slope of the land. Where the retaining wall is located, that wall will be knocked out and the addition added there as seen in the rendering.